The ink-jet printing process involves the ejection of fine droplets of ink onto a print medium such as paper in response to electrical signals generated by a microprocessor. Typically, an ink-jet printer utilizes a set of cartridges mounted on a carriage that is moved relative to the surface of a print medium.
Each cartridge includes a print head with an orifice plate that has small nozzles through which the ink droplets are ejected. Adjacent to these nozzles is an ink chamber where ink is stored prior to ejection. Ink drop ejection is typically achieved either thermally or piezoelectrically. In thermal ink-jet printing, each nozzle is associated with a resistor element. Each resistor element is in turn connected to a microprocessor, whose signals direct one or more resistor elements to heat up rapidly. This causes a rapid expansion of ink vapor that forces a drop of ink through the associated nozzle onto the print medium. In piezoelectric ink-jet printing, ink droplets are ejected due to the vibrations of piezoelectric crystals stimulated by electrical signals generated by the microprocessor.
The ink compositions used in ink-jet printing typically employ water, colorants, surfactants and water-miscible organic co-solvents. The colorants that may be employed include dyes or pigments.
Once an ink has been deposited on a print medium it takes a certain time for the printed image to dry. Drying is driven in part by evaporation and in part by absorption into the print medium. Short “drytimes” are generally preferred in the printing industry as they reduce surface bleed, reduce the potential for smudging, and shorten the overall print time.
The drytime of an image is influenced by a number of factors that include the chemical composition of the deposited ink and the physical and chemical characteristics of the print medium. In particular, the rate at which an ink is absorbed into a print medium is affected by the surface tension and viscosity of the ink. Inks with low surface tension and viscosity are absorbed more quickly into print media. Surfactants reduce the surface tension of inks and are therefore added for this purpose to most ink compositions. However, reducing the surface tension and viscosity of an ink also affects the jet velocity, droplet separation length, drop weight and stream stability of the ink. This is particularly true when inks are ejected at high frequencies, i.e., at high printing speeds. For example, inks with low surface tensions (e.g., less than about 30 dynes/cm) do not have the ability to fire normal drop weights at high frequencies—the drop weight usually decreases more than 50% at firing frequencies above 20 kHz. As a consequence, inks with short drytimes tend to lack stability at high firing frequencies while inks that are stable at high firing frequencies tend to exhibit long drytimes. This currently limits the speed of ink-jet printers. There is therefore a need in the art for ink compositions that are stable at high ejection frequencies and exhibit short drytimes when deposited on a print medium.